In general, since a tire has a geometry exhibiting symmetry of revolution about an axis of rotation, the geometry of the tire can be described in a meridian plane containing the axis of rotation of the tire. In what follows, for a given meridian plane, the radial, axial and circumferential directions refer respectively to the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire, and perpendicular to the meridian plane. In what follows, expressions “radially inside” and “radially outside” respectively mean “closer to the axis of rotation of the tire, in the radial direction” and “further from the axis of rotation of the tire, in the radial direction”. The expressions “axially inside” and “axially outside” respectively mean “closer to the equatorial plane in the axial direction” and “further from the equatorial plane, in the axial direction”, the equatorial plane being the plane perpendicular to the axis of rotation of the tire and passing through the middle of the tread surface of the tire.
The tread of the tire comprises raised elements separated by grooves. A raised element extends radially outwards from a bottom surface as far as a contact face, intended to come into contact with the ground when the tire is running, over a height H and comprises lateral faces. A groove is a cut in the tread, delimited laterally by lateral faces of raised elements facing one another and delimited radially towards the inside by a groove bottom resting on the bottom surface.
The tread of a tire comprises at least one first elastomer compound referred to as base compound.
An elastomeric compound is a material containing a diene elastomer of natural or synthetic rubber type, obtained by blending the various components of the material.
An elastomeric compound, after curing, can be mechanically characterized in particular by its elongation at break at 23° C. According to a first measurement method, the elongation at break is measured in accordance with French Standard NF T 40-101(December 1979), under uniaxial attention, on an “H2” test specimen 2.5 mm thick stretched until it breaks at a rate of 500 mm/min and at an ambient temperature of 23° C. According to a preferred second measurement method, the elongation at break is measured on a sample of elastomeric compound taken from the vulcanized tire. According to this second method, the sample taken is used to form a test specimen 18 mm long, 1.1 mm wide and 0.3 mm thick. The test specimen is taken from the tire in such a way that its length is perpendicular to the circumferential direction of the tire. This test specimen is then stretched on an extensometer at a rate of 50 mm/min at a temperature of 23° C. until it breaks. The elongation at break and the stress at break are recorded for at least two test specimens.
The tread of a tire needs to meet various performance requirements such as, by way of non-exhaustive examples, resistance to wear, resistance to mechanical attack, grip, rolling resistance. Because the raised elements and the grooves of the tread are subjected to different stress loadings, it is known practice of those skilled in the art that one way of differentiating these zones is to use elastomeric compounds which are specialized by zone.
Thus, it is known practise to use a second elastomeric compound, referred to as coating compound, superposed on the base compound and applied in particular to the lateral faces of the raised elements and to the bottoms of the grooves. Combining a base compound with a coating compound on the lateral faces and in the bottoms of grooves has been used, in the past, to achieve various objectives:                to improve grip, as described in documents EP 1499507, EP 1682360, EP 1682361,        to reduce rolling resistance, as described in document WO 2010137143,        to improve the ability of the groove bottoms to withstand cracking, as described in documents WO 1999037489, WO 2010072234.        
The coating compound may be present not only on the lateral faces and the bottoms of the grooves but also on the contact faces of the raised elements, as described in documents U.S. Pat. No. 1,719,628 and WO 2010072234. It should be noted that those documents disclose a coating compound of constant thickness, covering the entirety of the tread.